Variable gain amplifiers (VGAs) are used inter alia in receiving and transmitting paths of basestations for conditioning the amplitude of a Radio Frequency (RF) signal for various gain variations as signal level at the antenna or gain spread of the electrical components.
In the RF spectrum ranging from 0.7 GHz to 3.8 GHz it is very necessary to keep an almost constant and linear amplification with a signal to noise ratio as large as possible. These tasks are not trivial to achieve when such a large frequency range is involved.
Known prior art VGA amplifiers comprise a series connection of a small signal amplifier followed by an attenuator and further followed by a large signal amplifier. However, this known architecture generates intermodulation noise, the attenuator being hard to adapt to the amplifiers impedances in whole frequency range. An important parameter of any VGA is OIP3 i.e. third order intercept point (IP3) at the output. This is a measure for the small-signal third order linearity of the circuit. The OIP3, expressed in dBm, indicates the extrapolated power level at which the first harmonic has the same power as the third harmonic at the output.
The overall OIP3 at maximum gain of the VGA is limited by the OIP3 of large signal amplifier in a power amplifier as described above. This is no surprise because said amplifier should handle the largest signals in the circuit. It should also be seen that, at maximum attenuation, the OIP3 is fully determined by the small signal amplifier and the attenuator.
The problem with this OIP3 situation is twofold                1. the linearity at maximum attenuation is too low (18.4 dBm, see FIG. 1)        2. the linearity is not constant over attenuation and heavily dependent on the attenuation setting.        